1. Field of the Invention
The present invention relates to a semiconductor device including an oxide semiconductor and a manufacturing method thereof.
In this specification, a semiconductor device generally means a device which can function by utilizing semiconductor characteristics, and an electrooptic device, a semiconductor circuit, and electronic equipment are all semiconductor devices.
2. Description of the Related Art
In recent years, transistors that are formed using a semiconductor thin film having a thickness of about several nanometers to several hundreds of nanometers over a substrate having an insulating surface such as a glass substrate have been attracting attention. Transistors are widely used for electronic devices such as ICs (integrated circuits) and electrooptic devices. In particular, transistors are urgently developed as switching elements of image display devices typified by liquid crystal display devices and the like.
Further, transistors in which a channel formation region is formed using a metal oxide having semiconductor characteristics (hereinafter also referred to as an “oxide semiconductor”) are already known.
Transistors are roughly classified into a top-gate type in which a channel formation region is provided below a gate electrode and a bottom-gate type in which a channel formation region is provided above a gate electrode. These transistors are generally manufactured using at least five photomasks.
Further, in order to improve reliability, a liquid crystal display device in which a sensor portion is sealed with a sealing portion doubly is disclosed in Patent Document 1.